US 6680901 B1 Abstract In order to shorten delay time in a synchronization circuit in an OFDM demodulator, with respect to a preamble for estimating carrier frequency/channel characteristic, the synchronization circuit does not have a delay circuit and a frequency error compensation signal of a carrier frequency estimating circuit is supplied to a channel distortion estimating circuit connected to an output of an FFT. In the channel distortion estimating circuit, a carrier frequency deviation is compensated.
Claims(3) 1. An orthogonal frequency division multiplexing (OFDM) demodulator for demodulating an OFDM modulated signal, comprising:
a quasi-synchronous detector for converting the OFDM modulated signal into a complex baseband signal of two components by quasi-synchronous detection;
a synchronization circuit for receiving the complex baseband signal and establishing symbol synchronization;
a fast Fourier transforming circuit (FFT) for Fourier transforming an output of the synchronization circuit into signals of respective subcarriers;
a channel distortion estimating circuit for compensating a carrier frequency error on the basis of an output of the FFT and a frequency error compensation signal outputted from the synchronization circuit and estimating channel distortion;
a channel distortion compensating circuit for compensating channel distortion in the output of the FFT n the basis of an output of the channel distortion estimating circuit; and
a demodulating circuit for demodulating an output of the channel distortion compensating circuit every subcarrier,
wherein the synchronization circuit comprises:
means for estimating a symbol timing from the complex baseband signal;
means for estimating a carrier frequency from the complex baseband signal and outputting a carrier frequency error compensating signal;
means for multiplying the complex baseband signal by the carrier frequency error compensating signal; and
means for establishing symbol synchronization with the multiplication result on the basis of the estimated symbol timing.
2. An OFDM demodulator according to
3. An OFDM demodulator according to
Description The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) demodulator of a burst signal transfer system using an OFDM modulating method and, more particularly, to an OFDM demodulator in which a processing delay of a synchronization circuit can be shortened. An OFDM modulating method as a modulating method promising for a high-speed data transfer such as a high-speed wireless LAN has been being examined. A conventional OFDM demodulator for receiving an OFDM modulated signal will be described by referring to drawings. FIG. 4 is a block diagram showing the configuration of a conventional OFDM demodulator. FIG. 5 is a diagram showing a format of an OFDM burst signal. As shown in FIG. 5, at the head of each burst data In FIG. 4, an OFDM burst signal is received by an antenna A/D converters A synchronization circuit FIG. 6 is a block diagram showing the synchronization circuit The symbol timing estimating circuit On receipt of the preamble A delay circuit The complex multiplier After establishing symbol synchronization with the output of the complex multiplier A fast Fourier transform (FFT) circuit On receipt of the preamble The preamble As a result of estimation, the channel distortion estimating circuit The channel distortion compensating circuit FIG. 7 is a block diagram of the channel distortion estimating circuit A subcarrier demodulating circuit As described above, the conventional OFDM demodulator has the delay circuit According to the orthogonal frequency division multiplexing (OFDM) modulating system, data to be transmitted is divided into a plurality of low-speed subcarriers. The period of a symbol is therefore long (generally, about 4 μsec) and delay time in the delay circuit There is consequently a problem of a low throughput of a whole OFDM communication system due to the delay time in the delay circuit in the synchronization circuit. In order to solve the problems, according to the invention, there is provided an orthogonal frequency division multiplexing (OFDM) demodulator for demodulating an OFDM modulated signal, comprising: a quasi-synchronous detector for converting the OFDM modulated signal into a complex baseband signal of two components by quasi-synchronous detection; a synchronization circuit for receiving the complex baseband signal and establishing symbol synchronization; an FFT for Fourier transforming an output of the synchronization circuit into signals of respective subcarriers; a channel distortion estimating circuit for compensating a carrier frequency error on the basis of an output of the FFT and a frequency error compensation signal outputted from the synchronization circuit and estimating channel distortion; a channel distortion compensating circuit for compensating channel distortion in the output of the FFT on the basis of an output of the channel distortion estimating circuit; and a demodulating circuit for demodulating an output of the channel distortion compensating circuit every subcarrier. Specifically, a frequency error compensation signal is detected by using a preamble signal for estimating carrier frequency/channel characteristic in a carrier frequency estimating circuit in a synchronization circuit having no delay circuit. The frequency error compensation signal is outputted to a complex multiplier in a propagation path distortion estimating circuit. After phase rotation caused by a frequency error is corrected by an output of the complex multiplier, the channel distortion is estimated and compensated. As a result, the carrier frequency deviation is not compensated in the synchronization at the front stage of the FFT but is compensated after the FFT. The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein: FIG. 1 is a block diagram of an OFDM demodulator of the invention; FIG. 2 is a block diagram showing the configuration of a synchronization circuit FIG. 3 is a block diagram showing the configuration of a propagation path distortion estimating circuit FIG. 4 is a block diagram of a conventional OFDM demodulator; FIG. 5 is a diagram showing a format of an OFDM burst signal; FIG. 6 is a block diagram showing the configuration of a synchronization circuit FIG. 7 is a diagram showing the configuration of a channel distortion estimating circuit An embodiment of the invention will be described by using the drawings. FIG. 1 is a block diagram showing the configuration of an OFDM demodulator of the invention. The format of an OFDM burst signal is as shown in FIG. The A/D converters The synchronization circuit FIG. 2 shows the configuration of the synchronization circuit The symbol timing estimating circuit On receipt of the preamble At this time, in the conventional synchronization circuit After establishing the symbol synchronization by the symbol synchronization processing circuit With respect to data The fast Fourier transform (FFT) circuit On receipt of the preamble The carrier frequency is compensated by frequency error information from the synchronization circuit As a result of estimation, the channel distortion estimating circuit The channel distortion compensating circuit FIG. 3 is a block diagram of the channel distortion estimating circuit The subcarrier demodulating circuit In the embodiment shown in FIG. 1, as a quasi-synchronous detection circuit, a reception signal is first subjected to orthogonal component detection and a resultant is A/D converted. Obviously, on the contrary, it is also possible to perform the A/D conversion first and execute the orthogonal component detection. As described in detail, according to the invention, in order to shorten the processing delay time in the synchronization circuit, the compensation of the carrier frequency deviation of the preamble for estimating the carrier frequency and the channel characteristic is not performed in the synchronization circuit but is performed after the FFT. As a result, without providing the synchronization circuit with the delay circuit unlike a conventional manner, by using the process delay of the FFT, the process delay of the synchronization circuit can be largely shortened. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is, therefore, contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention. Patent Citations
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